Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Being obese is currently associated with about 14 percent of cancer deaths in men and 20 percent in women, compared with about 30 percent each for smoking, Dr. Walter C. Willett of the Harvard School of Public Health, told the annual meeting of the American Association for the Advancement of Science.

"As smoking goes down and obesity goes up it won't be long before obesity is the No. 1 cancer killer," Willett said at a symposium on cancer prevention.

Added Dr. Douglas R. Lowy of the National Cancer Institute: "Cancer prevention begins at home. ... Not all of us always act in our own best interest."

In the 1980s, researchers focused on the amount of fat people ate as a probable cause of cancer, but studies did not strongly support that. Later they turned to diets high in fruits and vegetables as a way to reduce cancer, but again, Willett said, they struggled to find convincing evidence in studies.

Now attention has turned to obesity, and more and more research is providing evidence that indict that as a cancer cause.

That does not mean people should stop eating fruit and vegetables and go to a high-fat diet, he quickly added. "We do see evidence of a benefit for heart disease, I think that's pretty real," he said.

And, he noted, studies have indicated some benefit from a high fruit and vegetable diet in some cancers, he added, including mouth, esophagus, lung and stomach.

Overall, Willett estimated 30 percent to 35 percent of cancers are due to nutritional factors, much of it to obesity.

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